Our multifaceted investigation on normal and atypical melanocytes concerns the following topics: 1. A study in vitro and in vivo of conjugates of MSH with cytotoxic agents on melanoma cells. 2. The mechanism by which cAMP increases the activity of tyrosinase. 3. The mechanism of internalization and of shedding of receptors for MSH. 4. The cytotoxicity of substances produced by the tyrosine-tyrosinase reaction. 5. The chemical nature of the receptors of MSH. 6. The culturing of normal melanocytes from guinea pigs and mice.